High-sensitivity detection of magnetic fields is critical to many applications including ordinance detection, geophysical mapping, navigation, and the detection of bio-magnetic fields associated with heart and brain activity. Conventional superconducting magnetometers based on superconducting quantum interference devices (SQUIDs) provide a high sensitivity for magnetic field detection but are bulky and require expensive cryogenic cooling. Atomic magnetometers, which are based on optical measurements of unpaired electron spin in an alkali metal vapor are being developed. These atomic magnetometers do not require cryogenic cooling and are capable of measuring the absolute magnetic field at high sensitivity (down to less than one femtotesla).
U.S. Pat. No. 8,212,556 (hereinafter, the '556 patent), which is commonly owned herewith, issued to P. Schwindt and C. N. Johnson on Jul. 3, 2012 under the title, “Atomic Magnetometer”. The entirety of the said '556 patent is hereby incorporated herein by reference. The '556 patent provided an atomic magnetometer in which a pump light beam and a probe light beam are directed in substantially the same direction through an alkali metal vapor cell. That arrangement was useful because, inter alia, it helped to reduce the lateral dimensions of the atomic magnetometer relative to other types of atomic magnetometers, and because it made it possible to sense magnetic fields at an arbitrary angle orthogonal an optical path of the pump and probe light beams. By contrast, other types of atomic magnetometers, in which the pump light beam is orthogonal to the probe light beam, are limited to sensing a magnetic field in a single direction.
The magnetometer of the '556 patent also utilized two different wavelengths for the pump and probe light beams. This arrangement allowed the pump light beam to be blocked with an optical filter while allowing the probe light beam to be transmitted through the optical filter and subsequently detected as a way to sense the magnetic field. The pump and probe beams were deliverable by either free-space or fiber optic transport. Hence the lasers used to generate the pump and probe light beams could be located at a distance from the atomic magnetometer.
The use of fiber optic delivery, in particular, was advantageous because it allowed a plurality of atomic magnetometers to be supplied with pump and probe light beams from a single pair of lasers in an arrangement in which the pump and probe light beams are respectively split and separately sent through optical fibers to each magnetometer.
In implementations described in the '556 patent, the vapor cell contains an alkali metal vapor (e.g. sodium, potassium, rubidium or cesium). The pump beam, which has a wavelength substantially equal to the wavelength of a first D-line atomic transition of the alkali metal vapor, is directed through the vapor cell to magnetically polarize the alkali metal vapor. Before entering the vapor cell, the pump beam, which is initially linearly polarized, is directed through a wave plate to convert it to circular polarization.
The probe beam, which is linearly polarized and which has a wavelength substantially equal to the wavelength of a second D-line atomic transition of the alkali metal vapor, is also directed through the vapor cell, where it undergoes a change in polarization due to a magnetic interaction with the polarized atomic vapor. Prior to entry into the vapor cell, the probe beam also passes through the wave plate, but the wave plate is dimensioned so as to operate as a quarter-wave plate at the pump wavelength and as a half-wave plate at the probe wavelength. Hence the effect of the wave plate on the probe beam is only to rotate its direction of linear polarization.
An optical filter that is located in the exit path of the pump and probe beams blocks the pump beam, permitting only the probe beam to impinge a photodetection system. The photodetection system comprises a differential pair of photodetectors onto which the exiting probe beam is directed by a polarization beam splitter. The photodetection system responds by generating an electric signal indicative of the change in polarization of the probe beam.
Useful information can be provided even by single-element photodetectors. However, as explained in the '556 patent, multi-element photodetectors can advantageously provide spatial discrimination by sensing the magnetic field at various locations within the vapor cell. In that regard, each individual photodetector element provides a respective spatial channel for sensing a respective portion of the volume within the vapor cell.
In practical implementations, we pumped and probed the vapor cell with a Gaussian beam having a relatively large full width at half maximum (FWHM) of about 15 mm. We made a four-channel magnetometer by impinging the exiting beam onto a pair of four-quadrant photodetectors. With this approach, we were able to achieve a spatial channel separation of about 5 mm.
Although the spatial resolution provided by such an approach is useful, an even greater range of applicability could be gained by further increases in the channel separation. Hence there remains a need for further advances that increase the separation between the spatial channels.